1. Field of the Invention
This invention relates to a support frame for electrolyte chambers in electrochemical cells with supported electrodes and in water depletion cells.
2. Description of the Prior Art
Electrochemical cells with supported electrodes are known. See, e.g., U.S. Pat. Nos. 3,471,336; 3,480,538 and 3,554,812. In such cells a porous support frame, which contains an aqueous electrolyte and defines the electrolyte chamber, is arranged between thin electrodes. Usually a cover layer is positioned between at least one electrode and the support frame. Such a cover layer is gastight when saturated with electrolyte and prevents any reaction gas from entering the electrolyte chamber. On the side facing away from the support frame or the cover layer, the powder electrodes are held by a net or screen which also serves as a contact. Additional screens support the reaction gas chamber.
The support frame in such electrochemical cells not only conducts the electrolyte through inlets and outlets, but also spaces the electrodes. In addition the support frame is permeable in two dimensions and allows a flow of ions from one electrode to the other. To perform the above functions, the support frame must be porous, but may consist of any desired material such as ceramic, glass, or plastic. Generally, the support frames are metallic screens and may have coarse pores in the middle region with fine pores in regions adjacent to the electrodes.
With respect to fuel cells, life tests have shown that perfect gas seals cannot be obtained since reaction gases such as hydrogen or oxygen pass into the electrolyte chamber from the gas chamber through small leaks in the cover layers. Reaction gas may also enter the electrolyte chamber by diffusion due to the difference in pressure between the gas and the electrolyte chamber. Such gases passing into the electrolyte chamber can degrade the operation of the cell even if only a small gas leak exists. Especially at higher current densities, gas losses of 1 ml/min per cell can create operational problems although such losses amount to less than 0.1% of the flow through.
The gases passing into the electrolyte chamber collect as bubbles which are held in part by capillary forces in the meshes of the screens serving as the support frame and cannot be removed, or only partially so, by the electrolyte flow. These gas bubbles increase the internal resistance of the cell, thereby causing a drop in cell voltage, and inhibit the current flow. If the gas distribution is not uniform and especially at large current densities such as 300 mA/cm.sup.2 and higher these gas bubbles cause non-uniform current density distribution and resultingly local overheating in the electrolyte and at the electrodes, leading to evaporation of water and further development of gas bubbles. In addition, the gas bubbles create an uneven flow of the electrolyte through the cell, thereby providing less local cooling and further heat build up.
Problems also arise in water separation cells (water depletion cells), due to gas bubble accumulation in the electrolyte chambers. Water separation cells, which provide for the removal of reaction water from the electrolyte of fuel cells, comprise an electrolyte chamber for the flow of an electrolytic liquid and at least one gas chamber separated therefrom by a diaphragm and into which water evaporates from the electrolyte chamber through the diaphragm. Generally, the electrolyte chamber and the gas chamber are supported by at least one screen, e.g., U.S. Pat. No. 3,783,107. If gas bubbles collect in the meshes of the screen present in the electrolyte chamber, freshly hot electrolytic liquid no longer arrives at these locations and the water evaporation rate decreases.